Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1()4(~ f5
This invention relates to a joint for anchoring a
structure to the sea bed, the invention being particularly
concerned with the anchoring of structures to be used in
connection with the extraction of oil and gas from deep
water sites, such as on the edge of continental shelves and
slopes, with particular respect to the North Sea and other
European waters.
Hydrocarbon deposits have been found to occur in
abundance under the continental shelves around the world
and during the last decade the North Sea and adjacent
waters have been found to overlie substantial oil and gas
reservoirs. Many oil and gas fields have been and are
being brought into production using existing as well as
- relatively new technology and these methods serve for
exploitation in water depths up to approximately 200m.
However, sedimentary basins suitable for oil and gas
reservoirs lie under deeper water down to depths of 3000m.
or more and there is thus a need for providing suitable
and economical methods for their exploitation.
Limitations of existing or currently proposed
systems mitigating against their use at depths greater than
200m. are as follows:-
Fixed Platforms - Steel ~ ;
- high cost of fabrication and installation.
- integrity dependent upon major piling systems
which cannot be proof tested.
- subject to corrosion problems and corrosion
fatigue with attendant difficulties in
maintenance and inspection.
- utilize a high proportion of highly skilled
labour and special steels.
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1(~4C~75
- do not provide oil storage facilities.
- impossible to instal module packages before float out.
Fixed Platforms - Concrete
- limited availability of suitable deep water construction sites.
- foundation problems and unsuitabilityfor certain sea bed
conditions.
- massive base structure reqùired for stability.
Floating Platforms and Semi-Buoyant Platforms
(Tension - leg structures)
- limited suitability for operation in northern North Sea
conditions.
- whilst possibly suitable for use at sites with a water depth
greater than 200m. there is a need to develop new and improved
anchors and mooring lines.
- difficulty of absorbing large movements on well flow lines,
risers, etc.
- disadvantage of un-protected conductor pipes.
Submerged Equipment (Sub-sea completions)
- limited amount of equipment and processing plant can be
installed,
- reduced accessibility for control, inspection and maintenance.
- problems of installation and completion of wells.
- difficulties of work-over operations.
- likely to be very expensive to install and operate.
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- require a surface platform of some kind in the vicinity.
- problems of pollution,
- potentially hazardous for sub-surface operating personnel.
Articulated Columns.
- limited amount of equipment and processing can be installed.
- reliance is placed upon very large mechanical movement joints,
with attendant maintenance problems.
Although the North Sea has been discussed above, the
invention is applicable to deep water sites throughout the world.
In accordance with one aspect of this invention there
is provided a joint for anchoring a structure to the sea bed
such that the structure can articulate; the joint comprising
two members connected together by flexible tendons such that
when the tendons are in tension the two members are held in --
closely adjacent, superposed relationship with a clearance
between the members sufficient to permit pivotal motion between
the two members, the tendons being of a synthetic material of
high strain capability, a first member of the two members
being adapted for location with respect to the sea bed and the -
second member of the two members being adapted for attaching
to the first member a structure that is to be anchored, said
structure to be anchored being buoyant enough to apply
sufficient tension to said tendons of synthetic material that
all of said tendons remain in tension during such pivotal
motion.
For a better understanding of the invention and to
show how the same may be carried into effect, reference will --
now be made, by way of example, to the remaining accompanying
drawings, in which:-
Figure 1 is a diagrammatic side view illustrating a
member that is to be locate~d with respect to the sea bed, and
pile installing equipment for driving piles so as to locate the
member. ~ `
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104(~5
Figures 2A and 2B are sectional side views of a detail
of the equipment of Figure 1 shown in two different operating
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10~ 75
conditionsO
Figures 3A and 3B are sectional side views of another
detail of the equipment of Figure 1, again shown in two
different operating conditions,
Figure 4 is a partly-sectioned side view of an
articulating joint, taken on the line IV-IV of Figure 5,
Figure 5 is a section taken on the line V-V of
Figure 4,
Figures 6, 7 and 8A are similar side views illus-
trating the joint of Figures 4 and 5 in combination with `
different forms of other structures.
Figure 9 is a sectional view of a detail of Figure 8,taken on the line IX-IX of Figure 10,
Figurè 10 is a sectional plan view of the detail of -
Figure 9, and -~
Figures llA and llB show various structures.
It is important to note that wherever water depths etc.,
are referred to herein these are only typical indications of
depths applicable, and it is probable that wide variations
could, in fact, be accommodated.
Referring first to Figures 1, 2A and 2B, and 3A and
3B, the member that is to be located with respect to the sea
bed is a prestressed concrete foundation structure 1 shown
resting on the sea bed and held in position by groups of
steel piles 2 driven through bores in the structure 1 by ;~
hydraulic drive equipment 3 generally of the type forming the
subject of Taylor ~Voodrow Construction Limited's British
Patent No. 966,094, in which in driving a group of piles load
is taken from driven piles of the group to a pile or piles being
driven. This operation is repeated successively using different
piles of the group so as to push the whole group into the ground.
The piling machine of Figure 1 includes eight large hydraulic
rams 4 mounted in two side-by-side groups of four on a thick
Cylinder 5. The cylinder, in addition to providing structural strength
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and serving as a crosshead connecting the hydraulic rams
can, if required, afford an atmospheric environment for
operating equipment therewithin. Each hydraulic cylinder
is connected to a steel tube or other structural shape which
forms the pile itself and can be remotely released at the
completion of the piling operation by pulling on release rods
6 (Figures 2A, 2B) to move a sleeve 7 upwardly first to
clear a segmented locking collar 8 and then to actuate levers
9 to move the segments of the collar 8 clear of abutting
flanges of the ram push rods 10 and the pîles 2. As shown
; in Figures 3A and 3B, as each pile 2 reaches its intended
depth a locking collet 11 thereon moves past spring-loaded
locking wedges 12 carried by the structure 1, the wedges 12
then springing into a locking condition (Figure 3B). The
pile tubes pass through the prestressed concrete foundation
structure 1 which rests on the sea bed, and the system can
be arranged so as to drive raking piles.
An alternative configuration uses the prestressed
concrete foundation structure as the structural support for
the piling equipment. In this method the hydraulic rams
are fixed to the piles and individually locked onto the top
of the concrete block. On the completion of piling the
hydraulic rams are released and recovered for further use,
utilising suspension cables 13. In this case, the hydraulic
equipment can be operated from a floating barge.
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The advantages of providing and locating the structure
1 in the way just described include the provision of a "proof-
tested" load carrying capability, the ability, if desired, to
disengage the structure from the sea bed and move it to another
location, remotely controlled operation and installation,
flexibility in design, and also that the structure is con-
structed and inspected in the dry.
Installation equipment will next be described. This
equipment includes guide lines for positioning the piling
machine. Where a large number of piles, necessitating many
re-positionings of the piling machine, have to be placed, the
number of guide lines used is minimised by, in turn, remotely
releasing guide lines from the anchor blocks and re-attaching
them at new locations for re-use. Also the piling machine can
be provided with mechanism such as hydraulic thrusters and
acoustic transmitters for guiding it into position. -
The installation equipment is operated from a surface
vessel which is preferably a purpose made floating barge. The i-
concrete structures 1 are constructed on shore and towed to
the site on pontoons, or they are self-buoyant. On site the
structures are docked and lifted by two 'portal' cranes on the
barge. After removing the pontoons (if provided) each structure
- is lowered to the sea bed by two main hoist ropes with four
guide lines attached. The position of the structure on the sea
bed is checked just prior to landing and any unacceptable error -
corrected by moving the barge. The eight piles are then lifted
into position in the well and held by a temporary jig prior to
installing the piling machine and locking onto the tops of the
piles. The assembly is then lowered using the guide lines until
the pile tips locate into the bores for the piles in the
structure. Using the dead weight of the equipment each pile
is pushed a limited distance into the sea bed and the piling
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operation is then continued by pushing one pile at a time.
When all the piles have been fully driven and located on the
top of the structure the piling machine is remotely released
and recovered by the barge to enable the whole operation to
be repeated.
When the equipment is used to provide a tension pile
system the flanges on the piles are pushed into contact with
the metal bearing surfaces connected to the liners of the
bores in the structure. The tensioned cable is anchored in
the structure itself. In the case of a compression pile system,
on the completion of driving, the piles are locked into the
structure by spring loaded locking wedges, and the annular
spaces are grouted solid.
The size and lengths of piles used are selected based
upon the requirements for a wide range of possible sea bed
conditions typical of, but not limited to, North Sea conditions.
In these, overconsolidated clays, with up to 60 t/m2 cohesive
shear strength and hard dense sands with angles of internal
frictions up to 45, have to be considered. In the configura-
tion described above the length of the piles can be variedbeforehand to suit a particular requirement but not during the
actual piling operations. However, it is envisaged that the
system could be modified to enable additional lengths to be
added during the piling operation. An alternative arrangement
would be to bore or jet out the piles and push smaller piles
through these to an increased penetration. It is to be noted
that the structure 1 is installed and the piles driven remotely
from a surface vessel. No manual intervention, or rigid guides
from the surface are required and the use of massive pile
hammers is eliminated. A principal advantage of the system is
that a "proof-loading" in both tension and compression is
clearly established for each pile, and permanent load bearing
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capacities determined with a high level of confidence.
Referring next to Figures 4 and 5, the articulating
joint therein illustrated is for deep water articulated columns.
The joint does not rely on mechanical pins or similar universal
joint type mechanisms. The principle of this joint is the use
of tensioned cables to provide completely flexible or moment
resistance type connections. The construction of the joint is
such as to provide the possibility of incorporating means of
access for men, plant and materials through the joint at
atmospheric pressure.
The joint is utilized to provide an articulated con-
nection of a column described hereinafter to the sea bed. An
articulated column, pivoting at the sea bed, has already been
foreseen as an attractive structure for operation in inter-
mediate water depths of between 200m and 500m. The main
principle of an articulated column is to permit motion in
sympathy with that of the surrounding water, resulting in a
substantial reduction in the forces and moments attracted to -
it. Its effectiveness therefore depends upon the effectiveness
of the joint or "hinge" at the column base.
In other published concepts mechanical "universal" type
joints have been proposed. These must rely for their efficiency ;
on the rotation of bearing ar.d continuous sliding low friction `
mating surfaces. Whilst such joints may be shown to be feasible,
they will necessarily be extremely large for the deeper struc-
ture and demand an excess of high cost skill in their develop-
ment and manufacture. Their viability with respect to durab-
ility, serviceability and in-service maintenance is questionable.
The present joint is based upon the following objectives: -
0 (i) The use of "lower technology" principles, both commen-
surate with the environment and already proven at a
scale which permits reasonable extrapolation.
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(ii) The avoidance of relative moving parts requiring
high technology and precision in manufacture, and
with sophisticated in-service maintenance aspects.
(iii) The provision for in-service inspection and possibly
replacement of critical components, but generally
having a low serviceability requirement.
The principle of the present joint is that the base
of the column 17 which it connects to the sea bed is held in
location by radially disposed tension cables. A typical con- `
figuration is shown in diagrammatic form in Figures 4 and 6
based upon calculations for a column operating in 500 m water
depth.
In principle, the joint consists of a lower concrete
ring beam 18 connected by a series of radially disposed inclined
terylene tendons 19 to an upper conical concrete "hub" 20.
The base of the articulated column 17 connects integrally with
the hub 20, which is located sufficiently clear of the ring beam
18 to permit maximum pivotal motion without the hub and beam
coming into bearing contact.
Under the action of environmental loads on the column,
the tendons permit full pivoting at the base whilst resisting
the resultant shear force. Bending stresses developed in the
tendons are limited to acceptable levels by radiused fairleads
21 within the hub and ring beam. The column is designed to
have sufficient excess buoyancy to maintain the tendons in a
state of normal tension throughout the range of motion which,
in the extreme storm conditions, would give an angular displace-
ment of 7 - 9. At 500 m depth variations in vertical force
would be small. The horizontal shear forces transmitted through
the tendons would be resisted by the ring beam, which as illus-
trated is in essence the structure 1 of Figure 1, that is it is
piled in position in the manner already described. Alternatively
it can be a gravity base. It is to be noted that:-
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sa) Whilst the tendons must be designed not to
exceed a limiting cycle stress from fatigue
considerations, the proportion of stress
resulting from shear, direct tension, and
pivoting can be varied by selection of component
dimensions.
b) By anchoring tendons to the central hub at two
or more levels, they can be made to generate a
moment/rotation. This may significantly reduce
column steady state angles with consequent buoyancy
savings, whilst limiting the extreme moment to
within that acceptable to the column base section,
and which may have been designed from other con-
siderations. This facility of interplay between
the root moment of the column and angular dis-
placement, does not exist with mechanical joints,
and could be important for columns operating in
shallower depths, where angular displacements are
larger.
c) The possibility exists of creating a central
penetration through the joint, which presents
the opportunity to obtain dry access via the
column to subsea facilities operating at one -~
atmosphere.
The ring beam and hub can be manufactured in normal
reinforced and/or prestressed concrete at a quality suited
to marine application. The tendons are manufactured from a ~
high strain capability material, for example "PARAFIL" (Trade ~-
Mark) or similar proprietory material. "PARAFIL" (Trade Mark) ~-
is an alkathene encased terylene tendon with a tensile capacity
50% of that of prestressing steels, but with an advantageously
low modulus of elasticity. The material has been developed
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and used for marine application, and has already undergone
considerable proving. In the present usage it is considered
to operate at a maximum stress in extreme conditions of only
30~ of its minimum tensile strength, so allowing for anchorage
efficiency, etc.
The ring beam and hub are constructed at a coastal
site and preassembled with the tendons. These are preset
at the required nominal direct tension by a subsequently
removable jacking arrangement acting between the ring beam
and the hub. The units can be either solid or cellular
concrete with minimum negative buoyancy, and are towed to
site using, specially constructed, recoverable buoyancy aids.
On location, the joint line assembly is lowered from
the buoyancy unit via cables to the sea bed. Depending upon
the configuration chosen, the ring beam is then either piled-
in (preferably in the manner already described), or ballasted
to provide an adequate gravity base. In Figures 4, 6 and 9,
where the ring beam constitutes a prestressed concrete
foundation structure as described with reference to Figures
1, 2A and 2B, and 3A and 3B, piles are diagrammatically shown --
at 22. The buoyant column 17, in stable vertical orientation
is guided by wire lines on to the nose of the conical hub,
and prestressed to act integrally with the hub, using conven-
tional steel tendons 23 (Figure 4). The method of making this
connection is shown in Figure 4. In this method, anchorages
24 for the steel tendons 23, with connectors 25 attached are
already located in the hub 20. After completing an elastomer
seal 26, the cavity between the column 17 and the hub 20 is
pumped dry and pressure caps 27 over tendon ducts 28 in the
column 17 are removed following which tendons 23 are installed
and stressed in the dry, and the cavity is grouted up.
Finally the temporary jacking arrangement tensioning the
terylene tendons 19 in the joint is removed, and the column
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buoyancy takes over, this being provided to a major extent
by a buoyancy tank 29. An alternative method of fixing the
buoyant column to the hub of the joint is to grout the
column into a sleeve constructed integrally with the hub.
In a further alternative the cables taper downwardly and
inwardly from a raised ring beam to support the foot of
the column below the ring beam, the column having in this
case a small negative buoyancy.
The articulated column can take variou~ forms.
In all cases the column constitutes a fixed facility for
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104~31~'~5
water depths generally in the range of 200M to 500M, and
can provide oil storage and/or vessel mooring and off-
loading facilities with some production facilities if
required. Oil storage where provided would be in the order
` of 500,000 barrels. In one form the column is a prestressed
concrete column being a cylindrical structure with integral
buoyancy chambers. As shown in Figure 6, the column 17 is
primarily a loading facility for tankers and is provided
with the mooring and loading equipment, on a deck super-
structure 30, necessary to provide for offshore loading
from an oil field which may not be served by a pipeline.
The column is in this case at atmospheric pressure and
serves to carry the various flow-lines from the production
unit. Operating water depths of up to or even greater than
500M are possible. There is no provision for oil storage.
As shown in Figure 8, the column such as shown in
Figure 6 (or it could be in the form of Figure 7) is con-
nected by the joint 18/19/20 to a housing 31 on the sea bed.
In all these combinations the basic concern is with mooring,
loading and storage. However, the same general combination
of components can be used to provide other facilities such
as support for a flare-stack. There is always a need for
some flaring, however small, and where sub-sea production
methods are employed a separate flare stack is required.
Even in cases where the production facilit~es-- are surface
mounted it is often desirable to provide a flare unit well
separated from the production area and the articulated
column structure is ideal for this purpose.
The column, manufactured in prestressed concrete of a
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quality suited to the marine environment, is constructed
at a coastal site in the horizontal orientation. The major
portion of its length is self-buoyant, but additional
buoyancy aids are required to support the lower sections
when afloat. The structure's stability during float-out
and installation is not sensitive to small load variations
and can incorporate a significant proportion of installed
plant. On`location, the column is set up into the vertical
position by controlled guying from external buoyancy aids
and possible additional ballasting.
Guide llnes already positioned on the previously
placed joint and held at the surface on buoys, are used to
guide the column to the joint nose and connection effected
as already described.
Referring again to Figure 8, and to Figures 9 and 10,
the housing 31 is constructed to house the well heads of
so-called "subsea completions" that are utilized in the
extraction of oil and gas, the housing containing chambers -
for housing subsea completions and that can be maintained
substantially at atmospheric pressure to permit man-access
to such completions. As indicated above, the housing also
serves as a foundation member for the whole assembly. -
The housing 31 is a prestressed concrete member which
is constructed to have with additional aid if required,
buoyancy and stability for towing from its place of
construction to its intended off-shore location, where it is
submerged to the sea bed. The housing 3~ then either rests
on the sea bed under the effect of gravity, or (and as illus-
trated) is held by piles 22 driven through passageways
provided in the housing 31 and into the sea bed, piling
being effected in the manner described above.
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The housing 31 has defined within it by walls 32
chambers for various purposes. These chambers include
chambers 33 for providing buoyancy and stability during
floatation and that are flooded for submerging, these
chambers also serving for housing well heads 34 of subsea
completions; chambers 35 for housing plant and/or oil
storage and providing passageways for oil/gas flow dùcts
from the subsea completions;a central chamber 36 from which
the chambers 35 radiate; and (where the housing 31 is to
form, as illustrated, part of an assembly in which the
col D 17 is held to the housing 31 by the joint 18/19/20)
chambers 37 giving access to anchorages of the tendons 19
of the joint. Such chambers can also be provided at the
base of the column 17. Tendons can be replaced by drawing
them into the housing 31.
The tendons 19 extend in four radially-spaced apart
groups from individual lower anchorages in a circular rib
38 on the upper surface of the housing 31, upwardly and
inwardly to individual upper anchorages around the hub 20.
The adoption of four groups of tendons gives a tendon
geometry such that no tendon is in line with a subsea
completion chamber 33, thus facilitating access to the
lower tendon anchorages from the chambers 37.
The interior of the housing 31 is connected to the
interior of the column 17 by an access shaft 39 that is
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104~75
' '
disposed cerltr.llly of the tclldoni. 19, ;Ind t~l~t pc~sses
throu~h se~l ~ssemblies between the shaft ~nd the housin~
31, and between the shaft and the hub 20.
One manner of utilising the equipment just described
is to submer~e the housing 31 with the joint; 18/19/20
fitted thereto at a desired off-shore loc~tion, and install
the piles 22. ~ith hatches 40 in the tops of the subsea
completion chambers 33 open, wells are drilled from a drill
ship with the drill shafts passin~ through the accessways
provided by o~ening the hatches, and throu~h bores in the
bases of the chambers 33. Once the wellhèads 34 hav~ been
installed, the column 17 is floated into position and
lowered to connect to the hub 20 of the joint, and this
connection made good. The deck superstructure 30 is then-
erected.
As an alternative, well drilling can be carried out
after installation of the column 17 and the deck super-
structure 30, with the wells bein~ drilled in the manner
ust described but from the deck. In Figure 8 a drill strin~
41 operated from the deck is shown.
A plurality of subsea completions can be housed in
each chamber 33, and in addition provision can be made for
drawing into the chambers 33 through normally sealed access
ports flowlines 42 from subsea completions disposed exter-
nally of the housing 31.
Flowlines 43 from the well heads 34 are connected to
risers 44 that run up the column 17. Articulated connections
to ~ive a three-joint system ~ provided in each run of
flowline 43/riser 44.
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In normal operation, the hatches 40 are closed, but
the chambers 33 are flooded, whilst the remaining chambers
to which access is required, or which contain plant, are
operated at atmospheric pressure. If it is desired to gain
access to the chambers 33 they are de~watered, and bulkhead
doors 45 to the chambers 33 are opened. The hatches 40 are
lifted off in the event of a well "blow-out" or abnormal
pressure occurring so that the structure of the housing 31
is not excessively loaded.
All flowlines, control lines and other connections are
made through bulkheads, and all bulkhead closures are
provided with two separate sealing arrangements.
Figure 8A shows an alternative form of the housing 31
in which a central part 31A is of a form described below
with reference to Figure llA, but having chambers 33 as
just described around it.
Further structures which are for operating beneath the
sea will now be described with reference to Figures llA and
llB.
Referring first to Figure llA, a vessel 45 is rigidly
fixed to the sea bed, mounted on a composite or pre-installed
foundation 1 that can be piled to the sea bed as already
described. An articulating column 17 as already described
is connected to the vessel 45 by the described joint 18/19/20
constructed to permit atmospheric access therethrough. Such
access can be supplemented by a capsule docking system. The
vessel 45 can be considered for water depths of up to 350m,
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104(~75
being a concrete sub-sea vessel desiglle(l fol thc one
atmosphcre enclosure of production e~luiplllellt a~ conxiderable
depth below the water surface and b~sed upon nuclear pre-
stressed concrete pressure vessel tc~cllnology. The vessel
is particularly suitab:le where t:he s~s~em iS used in
connection witll sub-secl. completiolls.
The vessel 45 is a large concrete strucl;llre always
experiellcill~, in operati.on, a general compressive :rield stress
and having a s~heri.cal inter~ .l avoid wh.icll can be ~Ised as a
production area, which is econc)micall.y at:tract.ive, and results
in a substantially uniform stress re~ime. The operational
loading is ideally suited for sucll a structure since it
derives substantially from the hydrostatic water pressure
imposed as it is submerged to its workillg depth, and it gives
rise to m0re or less uniform compression in the walls of the
structure. For a typical depth of 200m, a substantially
uniform compressive stress level o-f 11 N/mm is obtained and
this uniform state is in the main disrupted only by the
lateral wave forces and by t;he local loads andr!~m~?nt.s a,~lie~ by the
tower or column. The lateral wave forces can reasonably be `:
assumed to be sinusoi.dally distributed in plan, and they only
affect the field stress levels by appro~imately + 2.0
N/mm . The local stress disruptions arising frorn the tower
are in the order + ~ N/mm .
The 11 N/mm compressive field stress, which is a
satisfactory working level, is based on a minimum wall thick-
ness of 4 metres. This wall thickness is in turn ~erived
from empirical formulae based on results from long term
1 9 --
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implosion tests on concrete spheres and which takes account
of ultimate load requirements. The only steel required will
be relatively small quantities of prestressed steel and
normal reinforcement to take account of local effects.
In construction, the foundation structure where
separate is pre-installed as previously described and the
vessel is then lowered from a construction ring or buoyancy
raft to connect firmly with the foundation structure.
Referring to Figure llB, the vessel 45A herein
illustrated is for the storage and loading of oil in deep
water and is suitable for water depths up to approximately
350m, although greater depths could be possible. The vessel
45A is similar to the vessel 45 of Figure llA, carrying an
articulated column 17 which extends to the surface where it
carries a deck superstructure or platform 30, and resting on
a concrete structure 1 which is rigidly fixed to the sea bed.
The use of the articulated column carrying a platform near
the surface reduces overall horizontal forces. Since man-
access into the vessel 45A is not required, (and hence the
vessel is thus able to operate in water depths up to 500m)
it is not necessary that the joint 18/19/20 should in this
case permit man-access. An oil storage capability in excess
of 1 million barrels is envisaged.
In summary, the vessels 45, 45A are intended for
water depths in the general range of 300m to 500m, but the
system should be usable in appreciably greater depths. They
enable crude oil to be processed either from satellite sub-sea
completion systems or from conventional well conductors, or
both. The basic principle is to minimise the movements caused
and the forces exerted on the principal parts of the structure
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by waves, currellt, and wind by placing the bulk o~` thefacilities at a depth where such forces are significan~ly
lower than at, or in the vicinity o:f, the sea surIace.
manual operatiolls are carrie~d out at atmos~ eric prcss-lre,
and similarly atmospheric access Irom tl~e sur:race is
provided lor men, ~lant and materials. The prlnc.iple
extends present te~chllology, so lar as oil stora~e is concerned,
into deeper water. Storage volumes oL 1 million bbls and
over are possible, operating on l.lle water displacement
principle witll the vessel ~5 or ~5A always being lull of
either oil or water or both. The oil being lighter will
float on the water. Deep well pumps are provided to
enable oil or water to be pumped in or out as required. The
displacement water is passed through separators before being
discharged to the sea. In order to ensure complete tightness
against oil leakage the oil/water balancing system is arranged
so as always to provide a precompression in the vessel by
the external hydrostatic pressure.
A construction and installation procedure is as ~ollows.
The outer skin of the base part ol vessel is constructed in
a predredged dry colferdam. This is surrounded by a lloat-
able circular rein~orced concrete ralt, which forms a
construction "ring" or raft. A buoyant reinIorced concrete
base is also cons-tructed in the coflerdam and is preinstalled
before the vessel is towed to site. The cofferdam is flooded
and breached so tllat the raft and partly constructed vessel
with its base can be removed to a Stage 2 position. The
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. . .
.
~ 4~ 5
The ra.f-t thell serves as a construction base until
completion of the structure. It is also re(tuiretl as
a stability aid during the lat.t;er stages o:~ constrllction.
This is accomplished by a system oI guylines connecting
the vessel to the raIt, with the wllole con:L`igura-tion
maintained rigid by tlle tension in the guylines. 'l`his
arrangemellt can also be use~(l to limit construction and tow-out
draughts, utilising tlle ext;ra buoyallcy provided by the raft.
When the vessel has been towetl to its :fl.llal location it
is lowered from the buoyant circular enclosing raft to make
contact with tlle preinstalled foundation to which it is then
rigidly attached.
,~
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,,
, ~ . . , ,, ~ . :
